Multicolored outlines (For Blender Render)
With the Freestyle rendering option enabled, you will find the Freestyle Line panel showing up in the Material properties window.  The color selector in this panel allows you to choose a line color associated to the material.  The line color can then be referenced using the Material color modifier in the Freestyle Line Style panel in the Render Layers properties window.



At material boundaries, you might want to use the Priority parameter in the Freestyle Line panel to give a precedence to one of two competing line colors at material boundaries.  A post in the Freestyle development blog details this feature if you're interested.